Capacitors are the basic energy storage devices in random access memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices. They consist of two conductors, such as parallel metal or polysilicon plates, which act as the electrodes (i.e., the storage node electrode and the cell plate capacitor electrode), insulated from each other by a dielectric material.
The continuous shrinkage of microelectronic devices such as capacitors and gates over the years has led to a situation where the materials traditionally used in integrated circuit technology are approaching their performance limits. Silicon (i.e., doped polysilicon) has generally been the substrate of choice, and silicon dioxide (SiO2) has frequently been used as the dielectric material with silicon to construct microelectronic devices. However, when the SiO2 layer is thinned to 1 nm (i.e., a thickness of only 4 or 5 molecules), as is desired in the newest micro devices, the layer no longer effectively performs as an insulator due to the tunneling current running through it.
Thus, new high dielectric constant materials are needed to extend device performance. Such materials need to demonstrate high permittivity, barrier height to prevent tunneling, stability in direct contact with silicon, and good interface quality and film morphology. Furthermore, such materials must be compatible with the gate material, semiconductor processing temperatures, and operating conditions.
High quality dielectric materials based on ZrO2 and HfO2, besides others, have high dielectric constants, so are being investigated as replacements in memories for SiO2 where very thin layers are required. These high crystalline multivalent metal oxide layers are thermodynamically stable in the presence of silicon, minimizing silicon oxidation upon thermal annealing, and appear to be compatible with metal gate electrodes.
This discovery has led to an effort to investigate various deposition processes to form layers, especially dielectric layers, based on zirconium and hafnium oxides and silicates. Such deposition processes have included vapor deposition, metal thermal oxidation, and high vacuum sputtering. Vapor deposition processes, which includes chemical vapor deposition (CVD) and atomic layer deposition (ALD), are very appealing as they provide for excellent control of dielectric uniformity and thickness on a substrate.
Ritala et al., “Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources,” SCIENCE, 288:319–321 (2000) describe a chemical approach to ALD of thin oxide films. In this approach, a metal alkoxide, serving as both a metal source and an oxygen source, reacts with another metal compound such as a metal chloride or metal alkyl to deposit a metal oxide on silicon without creating an interfacial silicon oxide layer. However, undesirable chlorine residues can also be formed. Furthermore, zirconium and hafnium alkyls are generally unstable and not commercially available. They would also likely leave carbon in the resultant films.
Despite these continual improvements in semiconductor dielectric layers, there remains a need for a vapor deposition process utilizing sufficiently volatile metal precursor compounds that can form a thin, high quality metal oxide layer, particularly on a semiconductor substrate using a vapor deposition process.